This invention relates to antennas for radio frequency receivers and transmitters. More particularly, the invention relates to antennas and to the elimination of the undesired loss mechanisms which are typically associated with such antennas when coupled to a radio device.
In portable radio receivers such as paging receivers, the antenna is typically directly coupled to the first RF amplifier stage of the receiver via a coupling capacitor. An example of such an arrangement is illustrated in FIG. 1A which is a 3 schematic diagram found in U.S. Pat. No. 4,123,756, for a "Built-In Miniature Radio Antenna", issued to Nagata et al. The FIG. 1A schematic shows a loop antenna 10 directly coupled to an RF amplifier 20 via a coupling capacitor 30 therebetween. Unfortunately, at Ultra High Frequencies (UHF) and above, this structure has the disadvantage that various loss mechanisms act to reduce the efficiency of the antenna. For example, when the antenna is coupled to the receiver in this manner, alternative lossy paths exist between the antenna and receiver. One of the principal loss mechanisms is the undesired interaction of the antenna with other components on the receiver's printed circuit board, such as resonant circuits, shields and any conductors, for example. Thus signals are coupled from the antenna to the receiver circuits over paths other than the intended path through coupling capacitor 30. Unfortunately, the existence of these alternate lossy signal paths degrades antenna performance by lowering antenna efficiency. Moreover, antennas similar to the one just discussed are subject to self resonance when operation is attempted above UHF frequencies due to the loss mechanisms described above which prevail at these frequencies.
One approach which seeks to ameliorate the effects of these undesired signal paths is described in U.S. Pat. No. 4,491,978 for a "Portable Radio Receiver With High Gain Antenna", issued to Nagata et al. In that patent, Nagata et al. disclose an antenna 40 coupled to radio receiver circuits in the manner shown in the simplified schematic diagram of FIG. IB. That is, antenna 40 is coupled via a coupling capacitor 45 directly to the input of an RF amplifier 50 Amplifier 50 is coupled to a source of DC voltage via a high impedance circuit 55 and is coupled to ground via a high impedance circuit 56. The signal output of RF amplifier 50 is coupled to a mixer 60, the output of which is coupled to a high impedance circuit 57. Impedance circuits 55, 56 and 57 exhibit high impedances to the reception frequency, that is, the antenna circuit tuning frequency. In this manner, the Nagata et al. approach seeks to prevent undesired coupling between the antenna 40 and the receiver at the receive frequency by choking action. Unfortunately, with the Nagata et al. approach, only the receiver "back end" is decoupled from the antenna. That is, the RF amplifier, RF filter and RF mixer (receiver "front end") are still coupled to the antenna and subject to undesired loss mechanisms. The problem of undesired loss mechanisms between the antenna and portions of the receiver other than the receiver back end is not addressed by Nagata et al.